Thanks for the praise!
Yeah, that is of course one reason why the snow accumulates. However my train of thought was that all those depicted skeletons have zero need/motivation to get rid of the snow because they are mindless Undead. The Wight has a personality of his own but he doesn´t care because he won´t suffer from frostbite. The Lich however is vain and wants to present himself in the best fashion that´s why he has kept his clothes free of snow. Adventurers should get a good glimpse of him before he finishes them off.